Instead of conventional metal halide lamps or high-pressure mercury lamps, there is proposed use of a solid-state light-emitting device, particularly Light-Emitting Diode (hereinafter referred to as an “LED”) as a light source of a projector that is of an image display apparatus. The LED offers advantages in that miniaturization and low power consumption can be achieved in the projector, and it has a long life compared with metal halide lamps or high-pressure mercury lamps that are of conventional light sources. However, the LED has a smaller light-emission quantity. Therefore, as disclosed in Japanese Patent No. 3585097 (Patent Document 1), plural LED light sources may be arrayed to ensure a sufficient light quantity.
However, it is known that an available light flux is not increased in the whole optical system due to the following reason when the number of light sources exceeds a predetermined number. An etendue that is of a product of an area S in which the available light flux exists and a solid angle Ω is conserved in the optical system such as in the projector. Examples of optical modulators currently used in the projector include an LCD (Liquid Crystal Display), an LCOS (Liquid Crystal On Silicon), and a DMD (Digital Micro-mirror Device). There is a restriction to an effective angle in the optical modulators currently used in the projector, and modulation cannot be performed to the light flux that is incident to the optical modulator with an angle exceeding the effective angle. It is also difficult to expand the effective angle in devices such as a polarizing prism and a dichroic mirror that are used in the projector optical system. Therefore, there is the restriction in the solid angle Ω of the light flux that is effectively used in the optical devices. Further, it is clear that the area S of the light flux passing through each device depends on a size of the device. However, the area S cannot be increased in order to downsize the apparatus.
On the other hand, the etendue of the light diffused from the light source is proportional to the total area of the light source, that is, the number of light sources. However, when the etendue of the light source exceeds a minimum etendue of each optical device, the excess light flux cannot effectively be used. This is the reason why the available light flux is not increased even if the number of light sources is increased over a predetermined number.
Desirably the etendue of the individual LED light source is decreased as much as possible in the configuration in which LEDs are arrayed. For example, in a projector disclosed in Japanese Patent Application Laid-Open No. 2006-221954 (Patent Document 2), there is proposed an LED where an intensity distribution of the light exiting from the chip has directivity to decrease the solid angle of the light flux diffused from the light source, thereby suppressing the etendue.
In the projector optical system, generally illuminance homogeneity of a projected image is improved with homogenization means (integrator) for substantially homogenizing a light quantity distribution of the light emitted from the light source. In the projector disclosed in Patent Document 2, a rod integrator is used as the homogenization means. Light incident to the rod integrator is repeatedly total-reflected on inner surfaces of the rod integrator, and the light exits from the exit surface that is opposite to the incident surface. At this point, a surface light source having a homogeneous light intensity distribution is formed in the exit surface. In other words, the exit surface is illuminated in a superimposed manner with the light beams from virtual images of plural light sources that are formed across the incident surface by the internal reflection.
Patent Document 1: Japanese Patent No. 3585097
Patent Document 2: Japanese Patent Application Laid-Open No. 2006-221954
According to the configuration disclosed in Patent Document 1, a length L enough to homogenize the light quantity distribution of the rod integrator that is used as the homogenization means for homogenizing the light quantity distribution is expressed by Expression (3):L≧P/tan θ  (3)where P is a space between adjacent LEDs and θ is an angle from an optical axis at a position in which the intensity of the light emitted from each LED becomes a half of the intensity on the optical axis.
As is clear from Expression (3), the length L of the rod integrator depends on a divergent angle of the light flux incident to the rod integrator. Accordingly, as the directivity of the light flux is increased, the light flux is hardly dispersed and multiple reflections are hardly generated in the rod integrator. Therefore, the length necessary for the homogenization is increased. When the directivity of the individual light source is enhanced to effectively use the light flux while LEDs are arrayed, the rod integrator necessary to homogenize the light quantity distribution is enlarged, and therefore the apparatus is enlarged. Accordingly, the LED advantage of the miniaturization cannot be utilized.
In view of the foregoing, an object of the invention is to provide a compact illumination apparatus having a high directivity and a high homogeneity of light quantity distribution on the light-emitting surface and an image projector in which the illumination apparatus is used.